In recent years, the quantity of information (transmission capacity) transmittable on a single optical fiber is continually increasing due to an increase in the number of multiplexed wavelengths and higher optical signal modulation speeds. Increasing the information volume transmitted by way of optical fibers still further, requires boosting the utilization efficiency of the frequency band by devising a signal modulation method and cramming a large number of optical signals into a limited frequency band.
In the world of wireless communications, multilevel modulation (technology) makes high-efficiency transmission possible that exceeds a frequency utilization efficiency of 10. Multilevel modulation also appears promising for use in optical fiber transmissions and many studies have been made in the related art. In non-patent literature 1 for example, technology utilizing QPSK (Quadrature Phase Shift Keying) to perform 4-level phase modulation has been reported. In non-patent literature 2, technology for 32-level amplitude and phase modulation combining 4-level amplitude modulation and 8-level phase modulation has been reported.
(A) through (D) of FIG. 1 are drawings showing the signal point positions in the various modulation methods of the related art, and that describe the complex phase plane utilized in optical transmission. Signal points (complex notation on the optical electric field at the recognition time) for each type of optical multilevel signal point are plotted on the complex phase plane (also complex plane, phase plane, IQ plane).
(A) in FIG. 1 is a drawing for describing the signal points on the IQ plane. Each signal point can be displayed as complex coordinates (IQ coordinates) or polar coordinates shown as the amplitude r(n) and phase φ(n) in the figure.
(B) in FIG. 1 is a drawing showing an example of 4-level phase modulation (QPSK) signal point placement, and the four ideal signal points (symbols) utilized in transmitting multilevel signals are displayed on the complex phase plane Each of these ideal signal points is a fixed amplitude. The phase angle φ(n) is placed at the four positions: 0, π/2, π, and −π/2. Two bits of information (00, 01, 11, 10) can be transmitted per one symbol by transmitting one among these four symbols. Differential quadrature phase shift keying (DQPSK) that performs differential encoding beforehand is generally used if directly receiving (non-coherent receiving) this signal by using an optical delay detector however the signal point placement is the same for both (QPSK/DQPSK) so the present specifications do not differentiate between the two methods.
(C) in FIG. 1 is a drawing showing 16-level quadrature amplitude modulation (16 QAM) widely utilized in wireless (radio) communication. In 16 QAM the ideal signal points are arrayed in a matrix, and four bits of information can be transmitted per one symbol. In the example shown in the drawing, the upper two bit(10xx, 11xx, 01xx, 00xx) values are expressed as Q axis coordinates, and lower two bit (xx10, xx11, xx01 xx00) values are expressed as I axis coordinates. The distance between signal points can be increased for this signal placement and so (16 QAM) is known to provide high receiving sensitivity. In optical communications this type of quadrature amplitude modulation has been reportedly achieved by using a coherent optical receiver. The non-patent literature 3 for example, reports an experimental example for transmitting and receiving 64 QAM signals. Coherent optical receivers utilize a local light emitter source mounted in the receiver in order to detect the phase angle of the optical signal.
(D) in FIG. 1 is a drawing showing 16-level amplitude phase shift keying (16 APSK) in which 2-level amplitude modulation and 8-level phase modulation are superimposed. In 16 APSK, the 16 signal points are arrayed eight each in two concentric circle shapes having different amplitudes (symbol set is increased). A variety of signal point placements are in this way under evaluation for multilevel signals.
Studies are also being made on the other hand to boost the modulation speed on each wavelength (channel) to attain speeds ranging from 10 Gbit per second to 40 Gbit per second in order to expand the optical transmission capacity. However, increasing the modulation speed in this way causes the transmission quality to drastically deteriorate due to wavelength dispersion in optical fibers or non-linear effects such as the self-phase modulation effect. In the case of optical transmission, the effect from wavelength dispersion causes the optical transmission distance to decrease by half the square of the signal bit rate. So in optical transmissions at 10 Gbit per second or more, a dispersion compensator is needed for compensating for wavelength dispersion occurring along the transmission path between the end of the optical signal receiver and optical relay device. During optical transmissions at 40 Gbit per second for example, the capability of a typical dispersion fiber to withstand wavelength dispersion is a mere five kilometers. Adaptive compensation techniques that automatically limit degradation in signal quality to a minimum by utilizing a variable wavelength dispersion compensator mounted at the end of the optical receiver are now under evaluation.
However the variable wavelength dispersion compensator also presents many issues such as device size, complexity, cost, and control speed that must be resolved. In recent years, studies have been made of utilizing electrical stage compensation technology to estimate the received symbols by maximum likelihood sequence estimation (MLSE) and mounting electrically adaptive equalization circuits such as feed forward equalization circuits (FFE) or decision feedback equalization circuits (DFE) in the electrical circuits of optical signal receivers. However, wavelength dispersion compensation of electrical stages using the technology of the related art is incomplete since that technology only reforms the eye (I-plane) opening of the received optical waveform. The compensation effect was therefore at such an inadequate level that the receiver capability to withstand wavelength dispersion was effectively expanded just 1.5 to 2 times so that the transmission distance during normal optical fiber transmission at for example 40 Gbit per second only extended up to 10 kilometers.
One technology to resolve the aforementioned problems is for example the coherent optical electric field receiver system disclosed for example in non-patent document 4 (first technical example)
There is also on the other hand the phase pre-integrated type optical multilevel signal transmission system utilizing direct detection as previously proposed by the present inventors (second technical example). This scheme achieves a low cost, lower power consumption and also compact optical multilevel transmitter/receiver without utilizing a coherent detector and a detailed description is disclosed in patent document 1.
In patent document 2 serving as a third technical example the present inventors proposed an optical electric field receiver including a wavelength dispersion compensation function utilizing a delay detector.